Non-linear optical materials are typically employed for modulation of optical signals and for frequency conversion of laser light. Such non-linear optical materials generally comprise optically active groups which include a delocalised pi system connected on one side to an electron donating group and on the other to an electron withdrawing group. The term donor-pi-acceptor (DnA) system is often used in this context. When non-linear optical material is poled by the influence of an external electric field of force, a number of optically non-linear phenomena including frequency doubling and Pockel's effect are observed. By utilizing these phenomena, it is possible to employ the non-linear optical material in waveguiding components such as optical switches and frequency doublers. Non-linear optical materials which have been employed in electro-optic devices have, in general, been inorganic crystals such as lithium niobate or potassium dihydrogen phosphate. More recently, non-linear optical materials based on organic molecules, and in particular polar organic molecules, have been developed. The advantages of organic non-linear optical materials are their higher NLO activity on a molecular basis, their ability to provide very fast switching times in electro-optic devices, their low dielectric constant which enables higher electro-optic modulating frequencies to be achieved for a given power consumption and their ease of fabrication into integrated device structures, particularly when employed in polymeric form.
Some non-linear optical polymers are known in the prior art. For example, U.S. Pat. No. 5,208,299 discloses a variety of non-linear optical polymers derived from dihydroxyarylhydrazones. Such polymers may be polycarbonates, polyestercarbonates, and poly(hydroxyethers). In the examples a polycarbonate is described which is obtained by the polymerisation of a dihydroxyphenyl hydrazone and bisphenol A. Although these materials exhibit NLO activity upon orientation and have a relatively high glass transition temperature, their optical transparency has been found to be less than optimal. Japanese Patent Publication No. J-05-142,600 also discloses NLO polymers including fluorine-containing polyurethanes, polyimides, polyesters, polyamides, polycarbonates, and polyethers. These materials are said to exhibit NLO activity and have a refractive index which is easily controlled to match quartz-type waveguide media. Published European Patent Application No. 571 271 discloses second-order non-linear optical polymers and methods for producing them. Among the optical polymers mentioned are polysiloxanes, polymethacrylates, polyesters, polyurethanes, polyamides, polyimides, polyacrylates, polystyrenes, polycarbonates, and polyethers, as well as derivatives and/or copolymers thereof. The non-linear optical components are bonded to the polymer backbone. Our own copending European Patent Application No. 94202733 discloses non-linear optical polycarbonates which include a D.pi.A system comprising a donor bonded to an aromatic group, which aromatic group is bonded via a conjugated bond to an aromatic or conjugated cyclic group, which in its turn is bonded to the acceptor group. These materials exhibit a low loss of signal, good polability, and high Tgs. The main drawback to polymeric waveguides made from the above-described polymers is that they do not provide an optimal combination of properties. More particularly, such polymeric materials should have a high glass transition temperature, good polability, high stability of the Pockel's coefficient and minimal loss of signal. Although some of the foregoing polymers exhibit good properties in one or two of these areas, none of these materials provides an optimum combination of these properties.
The present invention provides an NLO polycarbonate with low loss of signal, good polability, high stability of the Pockel's coefficient and a high glass transition temperature.